The 3GPP Evolved Packet System (EPS) architecture is defined in 3GPP TS 23.401 and 3GPP TS 23.402. The Policy and Charging Control (PCC) architecture is defined in 3GPP TS 23.203. There are multiple network architecture scenarios depending on what type of accesses are involved, protocol options, roaming or non-roaming use cases, etc. These architecture scenarios are described in TS 23.401, 23.402 and 23.203. FIG. 5 shows an architecture diagram of a non-roaming scenario including accesses in the 3GPP family of accesses (e.g. E-UTRAN) and non-3GPP access, e.g. WLAN. Additional architecture options may be found in TS 23.401 and 23.402.
There are two architecture options for Policy and Charging Control (FCC); the “on-path” and the “off-path” architecture alternatives. The “on-path” alternative is used when the mobility protocol also supports QoS and bearer signaling. This is the case when GTP is used. The “off-path” alternative is when the mobility protocol does not support QoS and bearer signaling. This applies to Proxy Mobile IP-PMIP and host based Mobile IP (MIP). With “on-path” PCC there is only one policy enforcement entity, policy and charging enforcement function (PCEF). The PCEF is located in the PDN GW.
With “off-path” PCC, there are however two policy enforcement entities, the policy and charging enforcement function (PCEF) and a Bearer Binding and Event Reporting Function (BBERF). The BBERF is located in the Serving GW as well as in a network node in the trusted non-3GPP IP Access (The network node in trusted non-3GPP IP Access as an Access GW in the figure above). From now on, we refer to this network entity where BBERF is located using the generic term Access GW, or AGW. For each IP-CAN session, the PCRF has interfaces both to the PCEF (Gx interface) and BBERF (Gxa/Gxc interface). From now on, the Gxa/Gxc interfaces are commonly referred to as Gxx.
Based on information received in the messages on Gxx and Gx, the PCRF performs a so called “leg linking” to determine which Gxx and Gx sessions belong together for the same IP-CAN session. The sessions over Gxx are also referred to as Gateway Control Sessions (GCS).
When a certain event takes place in the EPS, for example an initial attach or a handover, the GCS is always created before the corresponding event is reported on Gx. For example, at initial attach the GCS is created before the Gx session is created. At handover of an existing connection to a new access, the GCS in the new access is created before the handover is reported on Gx. The PCRF can thus assume that the GCS is supposed to exist at the time when the Gx session is created or modified. Having this temporal order, GCS related signaling occur before corresponding Gx related signaling, simplifies the logic in the PCRF and thus eases the implementation efforts.
When PMIP is used in the network, the Proxy Binding Update (PBU) in most cases contains information (Handoff Indicator, or HI) to the PDN GW whether the PBU corresponds to a new attachment, i.e. creation of a new IP-CAN session or a handover of an existing IP-CAN session. When HI indicates a new attachment, the PDN GW creates a new PDN connection for the relevant user equipment (UE) and also a new IP-CAN session with the PCRF. A new IP address is allocated to the UE. When the HI indicates handover, the PDN GW instead “re-uses” the existing PDN connection and IP-CAN session, assigns the same IP address to the UE in the new access and sends an IP-CAN session modification to the PCRF to report the handover.
In case of handover from a 3GPP access technology to a non-3GPP access technology, there is however a special case where the state, initial attach or handover, is unknown and this is indicated in the PBU by setting the HI to “unknown”. This happens for example with UEs that have multiple network interfaces but cannot support IP level session continuity between those accesses. In this case, the PMIP RFC [RFC 5213] and 3GPP TS 23.402 allows for two alternative options:    1. The PDN GW can create a new PDN connection/IP-CAN session and assign a new IP address to the UE in the new access, or    2. Start a timer.    a. If the UE releases its IP address in the old access before the timer expires, the PDN GW can assume that it is a handover and in this case the PDN GW “re-uses” the PDN connection/IP-CAN session and assigns the same IP address to the UE in the new access.    b. If the UE has not released its IP address in old access when the timer expires, the PDN GW creates a new PDN connection/IP-CAN session and assigns a new IP address to the UE in the new access.
In case 2a, the current PCC procedures work fine. The GCS in the new access is created first and the PCRF can link the new GCS with the existing Gx/IP-CAN session. The problem occurs in case 1 and 2b. In this case the PCRF, upon creating of the new GCS, will link it to the existing Gx/IP-CAN session without knowing that a new IP-CAN session will be created later. The PCRF will thus link the new GCS to the wrong Gx/IP-CAN session.